In the arid and open areas of many current conflicts burying objects is a commonplace element of military tactics. The rationale is that this is often the single way of concealing them, and very simple to do in e.g. sandy terrain. These objects can be mines, concealed weapons or tunnels and bunkers. Correspondingly there is a strong requirement for efficient means of detecting these types of buried objects.
The circumstances and purposes for buried object detection vary. Still surveillance capacity linked to a high probability of detection is a general concern. For instance a military transport en route along a road must posses a possibility to detect the mines which may harm it when traveling at some reasonable speed. In contrast after a peace treaty there is very strong requirement for efficient demining requiring all mines to be found and deactivated. They may be spread over large areas, and not always in a fashion which is well controlled. In this case there is no real time demand though the surveillance task is often so large that surveillance capacity must be large. Searching for concealed weapons, is often delimited to certain areas and may not have any immediate real time requirement. However there may be a strong pressure to obtain results within definite deadlines so surveillance capacity is a concern in this case too.
An emerging application area is the restoration of former military storage and training areas to civilian land use. The areas can be severely polluted by unexploded ordonance, and harmful waste. The location of waste deposits may have been forgotten through the dramatic organizational changes in e.g. Eastern Europe.
When surveillance requirements are large the use of handheld mine detection devices would be inefficient. Also self-moving detection devices depending on magnetostatic or electrostatic effects (thus measuring the ground permeability or dielectricity constant) have low surveillance capacity. The reason is that static fields decline at short ranges, calling for careful and slow movements in the detection process. In contrast, radar is based on electromagnetic radiation. Since range attenuation of electromagnetic radiation is smaller than that of electrostatic fields, radar seems to be the principle to be preferred for large coverage subsurface object detection.
Subsurface objects may be small, and their signatures very weak. Therefore a detection device must sense only a small portion of the ground where the disturbance of the ground due to the presence of an object will be relatively noticeable. A problem with radar operating at larger surveillance ranges is therefore how to obtain sufficient resolution, isolating small volumes of the ground. The principle of synthetic aperture radar, SAR, is a well-known method to obtain high 2-dimensional resolution of the ground surface.
A Synthetic Aperture Radar, SAR, can be used from the ground and from the air. An airborne SAR produces two-dimensional images perpendicular to the aircraft path of flight. One dimension in the image is called range (or cross track) and is a measure of the “line-of-sight” distance from the radar to the target. Range measurement and resolution are achieved in synthetic aperture radar in the same manner as most other radars: Range is determined by precisely measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted pulse width, i.e. narrow pulses yield fine range resolution.
The other dimension is called azimuth (or along track) and is perpendicular to range over the ground surface. It is the ability of SAR to produce relatively fine azimuth resolution that differentiates it from other radars. To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam. The sharpness of the beam defines the azimuth resolution. Similarly, optical systems, such as telescopes, require large apertures (mirrors or lenses which are analogous to the radar antenna) to obtain fine imaging resolution. Since SARs are much lower in frequency than optical systems, even moderate SAR resolutions require an antenna physically larger than can be practically carried by an airborne platform: antenna lengths several hundred meters long are often required. However, airborne radar could collect data while flying this distance and then process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beamwidth results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna.
While this section attempts to provide an intuitive understanding, SARs are not as simple as described above. For even moderate azimuth resolutions, a target's range to each location on the synthetic aperture changes along the synthetic aperture. In SAR the energy reflected from the target must be “mathematically focused” to compensate for the range dependence across the aperture prior to image formation. When the aperture is large the SAR can give resolution near the radar wavelength which gives a sensitive focus and objects will vanish in the SAR image unless properly focused.
However, the previously known SAR cannot be used for underground detection since the electromagnetic energy cannot penetrate the ground sufficiently, but is reflected over the surface.
Hence, there remains a need for an improved radar that can be used for underground imaging.